Temperature drop when dissolving KH Buffer

[Ledo]

I've been using Aquaforest KH Buffer as my alkalinity part.

Kh Buffer - Aquaforest

KH Buffer by Aquaforest: a preparation for stabilizing carbonate hardness in the aquarium, supporting the maintenance of a stable pH and the health of corals.

aquaforest.eu

Aquaforest don´t tell us the composition.

I´ve noticed that when I mix the solution (80g of the product per liter of deionized water), it results in a temperature decrease.
For example: when using 400g of KH Buffer in 5 liters of RO/DI water, the solution temperature drops from 24 celsius to 20.5 (-3.5 celsius).

My question:

As far as I know, and I could be wrong, dissolving sodium bicarbonate in water results in an exothermic reaction. The same goes for dissolving sodium carbonate.
So what kind of reaction could be responsible for decreasing temperature?

 

[Randy Holmes-Farley Verified Community Expert]

Are you sure the solid was not cooler than the water when you added it?

I do not think it most likely that there are other components that override the enthalpy of dissolution of the sodium carbonate and bicarbonate, but they may add components such as potassium chloride which do take up heat when dissolving.

 

[Ledo]

Are you sure the solid was not cooler than the water when you added it?

I do not think it most likely that there are other components that override the enthalpy of dissolution of the sodium carbonate and bicarbonate, but they may add components such as potassium chloride which do take up heat when dissolving.

Solid and water are stored in same place.

I'll do another test tonight, taking note of this variable.

But this temperature drop is something that has always called my attention, because it is very noticeable, and lasts a long time, even with the beaker resting in a warmer environment.

I was wondering if some borate could be responsible.

 

[Randy Holmes-Farley Verified Community Expert]

Solid and water are stored in same place.

I'll do another test tonight, taking note of this variable.

But this temperature drop is something that has always called my attention, because it is very noticeable, and lasts a long time, even with the beaker resting in a warmer environment.

I was wondering if some borate could be responsible.

Yes, dissolution of borax is endothermic, but there optimally is not much present. Maybe they add an excessive amount for unknown reasons.

 

[Clarkjw2002]

I don't know the stoichiometry of the reaction but dissolving sodium carbonate/bicarbonate in RODI has always caused a drop in solution temperature for me. Always assumed that was part of the reason why it took longer to dissolve fully. Been mixing the tropic marin part B for years with this experience. Assume it is pure sodium carbonate/bicarbonate mix.

Dissolving part A (CaCl) causes a warming of the solution temp.

 

[Randy Holmes-Farley Verified Community Expert]

Ok, I've got this sorted out, despite many conflicting bits of info related to it on the internet.

Sodium bicarbonate takes up heat when it dissolves.

Sodium carbonate gives off heat when it dissolves.

More complex things can happen after dissolution (such as converting some HCO3- to H+ and CO3--), and looking at mixtures makes it even more complicated.

The upshot is that the cooling is from the sodium bicarbonate present.

This article takes measurements and discusses the molecular level mechanisms that impact temp:

https://pubs.acs.org/doi/pdf/10.1021/j100893a040

 

[EugeneVan]

I've been using Aquaforest KH Buffer as my alkalinity part.

Kh Buffer - Aquaforest

KH Buffer by Aquaforest: a preparation for stabilizing carbonate hardness in the aquarium, supporting the maintenance of a stable pH and the health of corals.

aquaforest.eu

Aquaforest don´t tell us the composition.

I´ve noticed that when I mix the solution (80g of the product per liter of deionized water), it results in a temperature decrease.
For example: when using 400g of KH Buffer in 5 liters of RO/DI water, the solution temperature drops from 24 celsius to 20.5 (-3.5 celsius).

My question:

As far as I know, and I could be wrong, dissolving sodium bicarbonate in water results in an exothermic reaction. The same goes for dissolving sodium carbonate.
So what kind of reaction could be responsible for decreasing temperature?

I am using Aquaforest KH buffer to make the component 2 solution for my dosing and I experience the same temperature drop too. I always warm up the RODI water to about 37 C before I start mixing the KH buffer with the RODI water. It take forever (over 30 min) for the KH buffer to dissolve in RODC water that is under 30 C.

 

[Ledo]

Sodium bicarbonate takes up heat when it dissolves.

Sodium carbonate gives off heat when it dissolves.

https://pubs.acs.org/doi/pdf/10.1021/j100893a040

Thanks Randy,

Well, I had to go to the Swedish website to read the article, but I confess that it didn't help much.
I trust you.
Interestingly, the information I gathered about the nature of dissolving sodium bicarbonate in water was about an exothermic reaction.
Good to know.

 

[Randy Holmes-Farley Verified Community Expert]

Thanks Randy,

Well, I had to go to the Swedish website to read the article, but I confess that it didn't help much.
I trust you.
Interestingly, the information I gathered about the nature of dissolving sodium bicarbonate in water was about an exothermic reaction.
Good to know.

Yes, that's what I first saw too, along with endless experiments for students that didn't actually give data. That's why I wanted to see an actual measurement.

 

[Saltyanimals]

I switched off using AF balling some time ago but had left over KH Buffer sitting around. I intended to use it to adjust alk but observed that while it will raise per their calculator, it doesn’t stay up. Alk will drop back down in a couple days. Anyone else experience this?

 

[Randy Holmes-Farley Verified Community Expert]

I switched off using AF balling some time ago but had left over KH Buffer sitting around. I intended to use it to adjust alk but observed that while it will raise per their calculator, it doesn’t stay up. Alk will drop back down in a couple days. Anyone else experience this?

That presumably just means the tank used it. Aside from pH effects, there’s nothing about any brand of buffer that makes it last longer. Buffers that boost pH will increase both biological and abiotic precipitation demand for alk and calcium.

 

[Saltyanimals]

That presumably just means the tank used it. Aside from pH effects, there’s nothing about any brand of buffer that makes it last longer. Buffers that boost pH will increase both biological and abiotic precipitation demand for alk and calcium.

I made the same assumption that it was simply consumed, but the odd patterns is the consumption also abruptly stops.
Example. I'm at 7.5 dKh. I add 15g of kH Buffer to take it to 8.5. Looks good after the rise. 2 days later back down to 7.5. No other changes to the CaRX or anything else to the tank. Had this experience multiple times. I thought maybe I was adding too fast and that caused a local precipitation somewhere. Slowing down didn't work either. That's why I started researching to see if this kH Buffer is anything different and should NOT be used to adjust alk but only used as part of the 3 part recipe. The latter part didn't make sense logically since alk rise is alk risk so overall strange.

I've only been able to keep my alk in the 8.5 range recently now that I've added an additional NaOH dosing as part of my pH experiments. Since I couldn't raise and not experience that mysterious retreat, I slightly over dosed NaOH to raise the alk as part of my NaOH tuning.

 

[Randy Holmes-Farley Verified Community Expert]

I made the same assumption that it was simply consumed, but the odd patterns is the consumption also abruptly stops.
Example. I'm at 7.5 dKh. I add 15g of kH Buffer to take it to 8.5. Looks good after the rise. 2 days later back down to 7.5. No other changes to the CaRX or anything else to the tank. Had this experience multiple times. I thought maybe I was adding too fast and that caused a local precipitation somewhere. Slowing down didn't work either. That's why I started researching to see if this kH Buffer is anything different and should NOT be used to adjust alk but only used as part of the 3 part recipe. The latter part didn't make sense logically since alk rise is alk risk so overall strange.

I've only been able to keep my alk in the 8.5 range recently now that I've added an additional NaOH dosing as part of my pH experiments. Since I couldn't raise and not experience that mysterious retreat, I slightly over dosed NaOH to raise the alk as part of my NaOH tuning.

I'm not really getting the issue.

Higher alk and higher pH increases the alk demand. Thus a boost naturally increases both types of demand, and demand drops again as the alk and pH return to normal.

 

[Saltyanimals]

I'm not really getting the issue.

Higher alk and higher pH increases the alk demand. Thus a boost naturally increases both types of demand, and demand drops again as the alk and pH return to normal.

Not so much of an issue, just an observation that I found it interesting that I can only temporarily raise it from 7.5 to 8.5 and it quickly drops back to 7.5 days later. It is like 7.5 alk is my tank's "happy place" The increase consumption could be an explanation if I boosted consumption with the alk and pH raise. However that new (boosted) demand should continue driving down alk instead it seems to stop when it got back to the happy place.

Non issue since I'm now able to keep it elevated with the addition of NaOH. Just interesting on the earlier struggle to keep it up. Made me think of those pH booster products that were sold. Sure it worked..but only temporary.

 

[Randy Holmes-Farley Verified Community Expert]

Not so much of an issue, just an observation that I found it interesting that I can only temporarily raise it from 7.5 to 8.5 and it quickly drops back to 7.5 days later. It is like 7.5 alk is my tank's "happy place" The increase consumption could be an explanation if I boosted consumption with the alk and pH raise. However that new (boosted) demand should continue driving down alk instead it seems to stop when it got back to the happy place.

Non issue since I'm now able to keep it elevated with the addition of NaOH. Just interesting on the earlier struggle to keep it up. Made me think of those pH booster products that were sold. Sure it worked..but only temporary.

IMO, that's not unusual. I wouid not describe the 7.5 as necessarily happy, just where demand slows.